1. Field of the Invention
The present invention relates to a display electrode substrate that is suitable for use in an active matrix display apparatus.
2. Description of the Prior Art
FIG. 9 is a plan view of a conventional display electrode substrate used for an active matrix liquid crystal display apparatus, showing a structure of the substrate per one picture element, and FIG. 10 is a sectional view taken on the line X--X in FIG. 9, wherein on a transparent insulating substrate 1 is formed a gate bus line 2 as a scanning line for the active matrix liquid crystal display apparatus, and part of the gate bus line 2 functions as a gate electrode of a thin film transistor (TFT) 8. A gate insulating film 3a of tantalum oxide (Ta.sub.2 O.sub.5) coats the gate bus line 2. A gate insulating film 3b is disposed throughout the surface of substrate 1 including the gate insulating film 3a.
A semiconductor layer 4 made of intrinsic amorphous silicon or the like is disposed at a position on the gate insulating film 3b that corresponds to the gate bus line 2. Semiconductor layers 4a and 4b made of n-type amorphous silicon or the like are selectively disposed on parts of the semiconductor layer 4. Also, the semiconductor layer 4 is a channel layer of TFT 8. On the gate insulating film 3b are integrally disposed a source bus line 5 and a source electrode 5a functioning as the signal lines at the active matrix liquid crystal display apparatus. The source bus line 5 is disposed to perpendicularly solid-cross with the gate bus line 2 so as to sandwich the gate insulating film 3b therebetween. The source electrode 5a is superposed on a lateral side of the semiconductor layer 4. A drain electrode 6 is disposed on the gate insulating film 3b in such a manner that part of the drain electrode 6 is disposed on the other side of the semiconductor layer 4.
Moreover, on the gate insulating film 3b is disposed a picture element electrode 7 made of a transparent conductive film and having an area corresponding to one picture element, the picture element electrode 7 being disposed on part of the drain electrode 6 to be electrically connected therewith.
The gate bus line 2, gate insulating films 3a and 3b, semiconductor layers 4, 4a and 4b, source electrode 5a and drain electrode 6 constitute the TFT 8. The TFT 8 functions as a switching element corresponding to one picture element at the active matrix liquid crystal display apparatus. The electrical characteristics of TFT 8 depend upon the width W1 of source electrode 5a and drain electrode 6 and the distance (channel length) L1 between the source electrode 5a and the drain electrode 6.
The TFT 8 and the picture element electrode 7 are disposed at every intersection of the gate bus line 2 and source bus line 5, in other words, as many as the number of picture elements disposed in a matrix form. A liquid crystal layer is sandwiched between the display electrode substrate thus formed and an opposite substrate on which the opposite electrode is disposed, thereby constituting a transparent type active matrix liquid crystal display apparatus.
The number of TFT 8 formed on the aforesaid display electrode substrate depends upon the number of picture elements. For example, for the liquid crystal display apparatus of 400.times.64 dots, 256000 TFTs 8 must be disposed in a matrix form. With the display electrode substrate having]such numerous TFTs 8, if even one TFT 8 among them causes a malfunction, the picture element corresponding to the defective TFT 8 becomes defective, so that the display quality of the liquid crystal display apparatus lowers. Accordingly, the entire numerous TFT 8 must be formed without any defect.
However, the process for producing the display electrode substrate is complicated as mentioned above, so that it is difficult to form numerous TFTs 8 on the insulating substrate of a large area without defects, which makes it lower the production yield of the display electrode substrate.
As an approach for improving the production yield of the display electrode substrate, it has-been proposed to provide two or more TFTs 8 per picture element so as to produce a redundant structure.
FIG. 11 is a plan view of the display electrode substrate of a redundant structure, showing a construction per a picture element, in which the components corresponding to those in FIG. 10 are designated with the same reference numerals.
The display electrode substrate is provided with a TFT 18 in addition to the TFT 8, corresponding to each picture element electrode 7, the TFT 18 being patterned simultaneously with the TFT 8 in the aforesaid manufacturing process for the display electrode substrate. Accordingly, the TFT 18 is constituted of the bus line 2, semiconductor layers 14, 14a and 14b, source electrode 5b and drain electrode 16.
The source electrode 5b at the TFT 18 is integrally disposed with the source bus line 5 and source electrode 5a, a portion of the source electrode 5b being disposed on one side of the semiconductor layer 14. A picture element electrode is disposed on one end of the drain electrode 16, the other end of the drain electrode 16 overlapping the other side of the semiconductor layer 14.
In the display electrode substrate, even when, for example, one of a plurality of TFTs 8 and 18 causes a disconnection that no current flows between the source electrode and the drain electrode although the gate electrode is switched on, so far as the remaining TFTs are in a normal state, the picture element normally operates, thus being effective to a disconnection of the TFT.
When two or more TFTs 8 and 18 are provided per one picture element as mentioned above, they are effective for a disconnection. However, since the number of TFT increases, the frequency of occurrence of the defect caused by short-circuit between the source electrode and the drain electrode at a TFT (to be hereinafter called the short-circuit defect) increases with an increase in the number of TFTs. Therefore, additional work is required to inspect the occurrence of short-circuit in the TFT so as to disconnect the short-circuited TFT from the picture element electrode.